This invention relates to hydraulic cylinder porting and piston configurations and especially to a unique special purpose piston ring which has no seal characteristics in one ring groove contact face thereby allowing virtual free fluid bypass in one piston direction proximate one end of the cylinder stroke.
In the prior art, the conventional approach to driving a hydraulic cylinder is by means of pressurized oil or hydraulic fluid which is admitted into the cylinder to exert its pressure against one piston face and thereby force the piston away from the pressurized oil input. When the piston has reached the end of its stroke, pressurized oil is admitted into its opposite cylinder end to act against the opposite piston face to drive the piston back toward the beginning cylinder end, with the first oil entry point or orifice serving as an oil exhaust during this piston stroke. Thus, in one case pressurized oil is admitted via an orifice in one end of the cylinder with the orifice at the other end exhausting previously admitted oil, and in the reverse piston direction the pressurized oil is admitted into the second orifice with the first orifice serving as an oil exhaust orifice. External valving accomplishes the directional flow control required to admit and relieve oil from the respective cylinder ends. The valves which therefore direct piston travel are typically operated by manual or automatic control to achieve powered motion of an apparatus connected to the piston rod. The cylinder oil orifices or ports are constructed either in the end faces of the cylinder or near the end faces, perpendicular to and through the cylinder bore wall.
The latter construction mentioned here offers an opportunity for means to a hydraulic cushioning effect in that the piston may be allowed to slide over its exhausting oil orifice as the piston nears the end of its stroke, thereby cutting off or trapping an amount of oil between the cylinder end wall and the driven face of the piston, and thereby greatly restricting the escape of the trapped oil being exhausted and preventing or greatly reducing the severity of the piston "bottoming" impact which can be damaging to both the cylinder and the machine being actuated by the piston rod. It has been customary in the prior art to use a metallic ring installed in a circumferential groove near either or both piston leading edges or faces to provide a more precise orifice closing medium than the piston surface itself. The ring therefore performs as an exhaust flow metering device in the orifice closing, covering or cushioning part of the cylinder stroke. The present invention provides a means for substantial performance improvement of the ring by offsetting the conventional ring's severe flow restriction, which causes retardation of cylinder reverse stroke response or "out of cushion" piston velocity. The invention concerns a metallic or rigid cross-section non-metallic hydraulic piston ring with conventional limited seal characteristics in one piston direction but which uniquely allows relative free fluid flow by-pass in the opposite direction.
In this type cushion, advantage is taken of the sectional strength and cross-section rigidity of the ring:
1. To resist high fluid pressure extrusion of the ring between adjacent high clearanced sliding members and into fluid exhaust orifices bridged by a portion of its circumference, as indicated above.
2. To preclude the perfect conformity seal characteristics of low durometer of soft seals and which results in a limited but positive circumferential bypass of fluid between its contact surfaces and those of its companion piston and cylinder bore.
Because of this characteristic fluid by-pass, leakage or "weepage", the ring has been used as an exhaust flow metering device in hydraulic cylinder cushions of the sidewall orifice type referred to above and discussed more fully below. While the amount of by-pass will vary with specific ring designs, surrounding surface contact and finishes, fluid viscosity and the amount of pressure differential present, the by-pass is generally adequate for the cushion purposes here described, as the piston and its leading edge ring progressively covers and bypasses the fluid exhausting orifice near the terminal end of the cylinder piston stroke. The volume of fluid so cut off or trapped is dependent on individual design requirements and is determined by the bore diameter of the cylinder and distance the orifice is spaced from the terminal position of the piston. In any regard, the combination of the piston momentum and the orifice closing process results in substantial increase of the now trapped volume pressure, providing a desired time-pressure resistance to the driven face of the piston thereby decellerating piston velocity and dampening otherwise damaging cylinder stroke end impact, as exhaust fluid being expelled by the leading face of the piston is greatly restricted in its escape. While this type of cushioning is quite common, its fluid flow restriction also greatly limits the reverse cylinder stroke response and stroke velocity until the piston and particularly the said ring have moved clear of the orifice which has been externally valved to supply input fluid flow for the opposite direction stroke. In many cases, this "out of cushion" time delay is objectionable, and in some cases a by-passing secondary "starter" circuit is added to offset it. The purpose of the invention is to eliminate the need for such secondary circuits or to improve cylinder performance where the secondary circuit need may be marginal or cost or space limited. The invention is an economical alternative by use of a piston ring that essentially seals (restricts) in one direction only, in effect creating its own secondary circuit within the piston itself, in many cases, by mere substitution of the inventive ring for a conventional ring. In addition, the typical secondary circuit mentioned is essentially a check valve which not only adds to the dollar cost of the assembly, it adds to overall assembly complexity, requires additional space (which can be critical), is an additional service consideration and presents an additional potential fluid contamination trap, all of which are avoided by the present invention.
Put another way, the ring provides the means for a less expensive, simpler, cleaner, more compact, more basic overall cylinder product, with far greater application flexibility in standard piston forms.
Typical piston rings used in hydraulic cylinders may be installed in piston grooves with either vertical face nearest the fluid pressure source since little or no difference can be detected in the amount of fluid bypassed from either face. This has an advantage in that a ring cannot be installed "backwards". This "advantage", however, has no relevance here since the purpose is to create a maximum fluid by-pass difference between ring faces that will permit maximum free fluid flow from its seal opposite vertical face, thereby greatly reducing time delay in reverse stroke response from the terminal piston positions as explained above. In most cases, applications of rigid piston rings in hydraulic cylinder pistons involve several rings in spaced grooves in the piston circumferential surface, the purpose being to offset individual ring leakage by use of multiples and thereby accomplish an acceptable total seal between the cylinder bore and the mating piston surface. The purpose of this invention, however, is to utilize the bypassing characteristics of a single ring, and to use it as a one-directional sealing, flow metering device for cylinder end cushioning purposes while relying on a "bi-directional" inboard seal ring, either conventional metallic or elastomeric, as the primary piston-bore seal in the overall cylinder stroke, with the combination creating a check valve effect. The invention provides an improvement in the subject ring in that it essentially self-cancels itself in one fluid flow direction while retaining its characteristic "seal" capabilities in the opposite direction.